By use of temperature-jump relaxation technique, we shall extend our studies of the neurophysin-binding system to analogs of oxytocin and vasopressin. We shall use analogs with amino acid substitutions and analogs which correspond to the first two or three amino acid residues present in oxytocin and vasopressin. Analysis of these systems should provide further insight into the role of various amino acid residues in the binding process. During this year we hope to complete the studies of metal ion binding to oxytocin and vasopressin. Both thermodynamic and kinetic information will be gathered. We shall utilize pH titrations to determine the binding constants and stopped flow temperature-jump to determine the mechanism of reaction. Metal ions are required for the interaction of oxytocin with both mammary and uterine receptor sites. Preliminary studies indicate that these metal ions (Co ion 2, Mn ion 2) affect the binding constant only and do not alter the number of receptor sites. After determination of the effects of metal ions on the binding, we shall examine the effects of the metal ion on the association and dissociation of the hormone-receptor complex. This should provide insight into the nature of the role of the metal ion. We shall examine the association and dissociation rates of oxytocin binding to its uterine receptor. The rate constants for oxytocin antagonists will be studied to determine whether or not the oxytocic response is due to receptor occupation or follows Paton's rate theory. BIBLIOGRAPHIC REFERENCES: Lymangrover, J.R., A.F. Pearlmutter, R. Franco-Saenz, and M. Saffran "Transmembrane Potentials and Steroidogenesis in Normal and Neoplastic Human Adrenocortical Tissue", J. Clin. Endo. Met., 41:697 (1975). Pearlmutter, A.F., E. Rapino and M. Saffran. "The ACTH-Releasing Hormones of the Hypothalamus Requires a Co-Factor", Endocrinology 97:1336 (1975).